The federal government and other authorities regulate allowable exhaust emissions from gasoline and diesel engines for automobiles, trucks, and other vehicles, such as off-road construction or agricultural vehicles, in an effort to reduce pollution. In order to ensure compliance with these regulations, the exhaust gases of these engines must be tested or otherwise analyzed for undesirable combustion by-products, such as hydrocarbons, carbon monoxides, sulphates, and/or oxides of nitrogen. In general, testing is accomplished by introducing exhaust gases, diluting these exhaust gases with clean air, and obtaining samples after the exhaust gases and dilution air are properly mixed.
Dilution tunnels are one known type of device for collecting, diluting, cooling, and mixing exhaust gases with filtered, ambient, and conditioned air in a ratio of gas to air for sampling and analyzing. At one end, a typical dilution tunnel has one inlet for receiving exhaust gases and another inlet for receiving the filtered air. An orifice plate is typically placed downstream of the exhaust gas and air inlets in order to induce turbulent flow and facilitate mixing of the exhaust gas and diluting air. Downstream of the orifice plate a probe is located in the tunnel to collect a sample of the mixture for analysis. One problem with placing orifice plates, or other obstructions such as inlet pipes, in the stream is that they tend to collect and remove some of the particulate matter from the stream, which distorts the downstream samples. Moreover, abrupt changes in tunnel geometry may also cause particulate matter to collect on the tunnel walls. This particulate matter may build up on the plate or walls over time and then periodically flake off, further distorting the samples taken downstream.
Full dilution tunnels collect and dilute the entire exhaust gas flow from the engine being tested. Current EPA regulations recommend that the dilution tunnels be sized to permit development of turbulent flow (Reynold's number greater than 4000) and obtain a homogeneous mixture of the exhaust and dilution air at the sampling location. Depending upon the engine displacement, in order to meet this requirement, a typical full dilution tunnel diameter may be on the order of 203 to 610 mm (8 to 24 inches) and a typical tunnel length, which is usually ten times the diameter, may be on the order of 2032 to 6100 mm (80 to 240 inches). The tunnel diameter and length is sized to insure proper mixing of the exhaust gas with the diluting air prior to taking the sample. The larger the engine displacement, the larger the tunnel diameter and tunnel length must be to accommodate the greater flow of exhaust gas. Thus, dilution tunnels for large displacement engines may be very bulky, even taking up entire rooms.
U.S. Pat. No. 5,090,258 discloses a multiple flow-dividing dilution tunnel system. Dilution air is introduced at one end of the tunnel. Further downstream, a portion of an exhaust gas stream is introduced into the tunnel via an exhaust gas inlet pipe inserted into the stream of dilution air. Further downstream, but proximate the end of the exhaust gas inlet pipe, a plurality of nozzles are inserted into the stream of dilution air/exhaust gas for introducing more dilution air, in this instance, a controlled amount of pressurized dilution air. This introduction of pressurized dilution air is used to properly maintain the ratio of the rate of the divided exhaust gas introduced into the dilution tunnel via the exhaust gas inlet pipe to the rate of the total gas introduced into the system, even during pressure loss fluctuations. One drawback of this system is that it requires a complex control system for reacting to pressure fluctuations. Another drawback is that it also requires a complex system of piping, both for splitting the exhaust gas stream and for introducing the pressurized dilution air. A further drawback is that the exhaust inlet pipe is exposed to the dilution air stream prior to the exhaust gas exiting the inlet pipe, causing undesirable cooling of the exhaust gas prior to mixing.
The disclosed method and apparatus for mixing streams of gases solves one or more of the problems set forth above.